Wiring a whole house fan to an attic fan

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Old 09-19-10, 10:53 AM
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Wiring a whole house fan to an attic fan

I have a power attic fan controlled off a thermostate in the attic. It comes on and off automatically. I have just installed a 2 speed whole house fan and would like it to operate in conjuction with the attic fan. I would also like to have the attic fan remain automatic when not using the whole house fan.

Both fans have their own curcuits. i am doing this to increase the eff. as I have only 2 gable vents and soffit vents and no ridge vent.

Any ideas on a scematic for a novice electrician would be helpful
 
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Old 09-19-10, 11:57 AM
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Just to be clear:

You want the attic fan to run at the same time when the whole house fan is running.

You want the attic fan to run as normal, automatically off the thermostat, even when the whole house fan is not running.

Correct?
 
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Old 09-19-10, 01:58 PM
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Like Scott, I have questions. What will you gain by running the whole house fan in conjunction with the attic evacuation fan? It will surely over power it in a few seconds making it useless. I would do away with the roof mounted fan as its life expectancy is relatively short. You can install the whole house fan to evacuate the air in the house by drawing fresh air in through partially opened windows if you want. Or you can install a gable mounted fan to evacuate the heat from the attic via the soffits. I don't think you want to use both. We'll wait for clarification.
 
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Old 09-19-10, 03:45 PM
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Yes I would like the attic fan to run when the whole house fan is running. The attic fan has been in the house since I bought it. (approx. 7 years). The whole house fan I installed recently.

I want the attic fan to coninue to operate normally. When the attic temps are above 93F it turns on.

My issue is that the whole house fan kicks off after about 15 to 20min due to the motor getting hot. I installed half round gable vents to help with the venting when the WHF is running. It extended the running time to about 30 min. The manufacture of the WHF says I am pressureizing the attic and need to increase my venting capacity. Hence turning on the attic fan when I run the WHF.

The belt is proper tension and not causing a problem.
 
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Old 09-19-10, 05:05 PM
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It doesn't matter if the vent fan is on when WH fan is because the WH fan will push the air through the vent fan opening. In fact it will push even more air then the vent fan. You are opening a few windows in the house a few inches aren't you?

Frankly it sounds like a bad motor. If you own or can borrow an Amprobe it would be interesting to check the amps the motor is drawing. Does the motor have a run capacitor? Please gve us all the information on the motor nameplate.
 
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Old 09-19-10, 05:29 PM
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Ray

Yes all the windows are open. I thought the same about the attic fan opening also.

A little more background info for you as well. I didn't think I would have to get into this so I tried to make my initial post what I needed but here it goes.

When I installed the WHF it would cut off after the time I told you. The motor has since quit working all together and when I contacted the manufacture they are replacing the motor for free and suggested I hook the attic fan up as the attic venting doesn't seem to be good enough. Hence my question on wiring together.

As with you I believe my venting is ok but to apease them I am working on using the attic fan for that purpose. It is possible the motor was bad from the get go and that when the motor comes in I won't have any problems.

Should I just wait and see if the motor solves the problem before I try and hook the two together?
 
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Old 09-19-10, 07:10 PM
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Definitely wait until the motor comes in. Trying to use the opening of the attic evacuation fan as an air escape place won't need that fan. The WHF will push it out as mentioned by Ray. Adequate gable venting in your case, as well as the soffit vents, should give less restriction. Let's don't fix it until we are certain it's broke.
 
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Old 09-19-10, 08:17 PM
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To go back to your original question, you will need a relay that is wired in parallel to the thermostat of the attic fan triggered with the switch leg of the WHF. I'm not the best with relays but we like to use RIB (relay in a box) relays. They are self contained and easy to connect. Functional Devices, Inc.
 
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Old 09-19-10, 09:03 PM
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While I don't believe the vent fan needs to be wired to run when the WH fan runs as Tolyn wrote you probably need a relay. A CT operated switch (relay) could be paralleled with the thermostat wires of the vent fan so the vent fan operates any time the WH fan runs. A place selling wood shop accessories would have them for turning on dust collection vacs when tools are turned on. They could probably be used for this purpose also.
 
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Old 09-19-10, 09:29 PM
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The manufacture of the WHF says I am pressureizing the attic and need to increase my venting capacity
I think you should check the whole house fan literature for required square inches of venting from the attic for this fan. You need more vents.
 
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Old 09-20-10, 05:33 AM
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I agree that I shouldn't fix it until I know its broke. Never crossed my mind to use a relay, shows how much of a novice I am.

Concerning the venting, before I added the gable vents when the fan was on I could hear air rushing through the soffit vents. I had calculated the sq inches with the attic fan hole and soffit venting and thought I had enough then. I installed the biggest gable vents Menards carries and the air rushing through the soffit vents is not audible now. With the gable vents, soffit vents and hole for the attic fan I have more than enough for sq inchs of venting capacity. The WHF manu. told me I needed to reduce the gable vents in the calculation to half to be more accurate (which I don't agree with). Anyhow, since I cannot add more venting without cutting into the roof they suggested to power up the attic fan when I run the WHF.

Thank You for all your suggestions and comments. First time I have been here and I will be a regular now.
 
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Old 09-22-10, 12:16 AM
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The point you (and obviously the WHF manufacturer, who should know better) are missing, is that your attic power ventilation capacity has zero bearing on the WHF. The total vent area does. If the attic is in fact being pressurized, you need more vent area. No amount of power ventilation that you could practically install can compensate for insufficient vent area. Your average WHF moves anywhere from 4,000-10,000CFM (a properly sized WHF should completely exchange the air in the house about once every two minutes). The most powerful attic fans are somewhere in the 2500CFM range.

For example, if you have a 1,500sqft house with 8ft ceilings, that's 12,000cuft of air volume. That would require a 6,000CFM WHF (remember, 2 minutes to completely exchange the air), and would also require at least 16 square feet of vent area in the attic (entire house air volume /750= minimum vent area in sqft). Any vent opening that is screened (mesh soffits count as screened) reduces its actual vent area by 50% when calculating for a fan. So your soffits and gable vents count for only half the area you had originally figured (which is what the WHF mfr told you). Re-calculate your vent area and see if you have at least 16 square feet.

If you come out under 16sqft, then your only option is to install extra roof vents or install a ridge vent. Running the attic fan isn't going to do a bit of squat in this situation. You can only force so much air through a given size hole no matter how much power you put behind it. If you need more air moving through, you need to cut another hole or make the existing hole bigger.
 

Last edited by JerseyMatt; 09-22-10 at 12:34 AM.
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Old 09-29-10, 10:31 AM
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Got the new motor in and wired it up. So far no problems. I have had it running for a maximum of about 1 hour so far and not kicking out anymore. I haven't had time to check motor temp yet after an extended run but that will be my next step.

JerseyMatt

In hind sight I agree with your assesment that powering the attic exhuast won't help me. I didn't think about it until you mentioned. Thank You. You saved me some money and time I did not need expend. I did the calculations and I am slightly lower than what I need if I cut the gable vents to 1/2 of manufactures claims. The hole for the attic fan, soffit vents, and gable vents is enough to exhaust about 5000cfm with a 5270 cfm WHF.

Since my motor is not over heating I think I can wait until next year when I reroof my house and put in the added venting with a ridge vent.

Thank You all for the advice. I look forward to coming backwith more problems in the future for advice.
 
 

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